1. Field of the Invention
This invention relates to a method for producing a foil material wherein a plastic foil is provided with a lacquer layer having variable viscosity, and a relief structure is then embossed into the lacquer layer. The invention further relates to an apparatus for producing a foil material having at least a device for applying a lacquer layer of variable viscosity to a plastic foil and a device for embossing the lacquer layer.
2. Related Art
Optically variable devices such as holograms, diffraction gratings, etc., have been used for some time as elements for protection from forgery or copying due to their optical properties varying with the viewing angle. For mass production of such elements it is usual to produce so-called xe2x80x9cmastersxe2x80x9d having the particular phase information in the form of a three-dimensional relief structure. Starting out from this master one produces by duplication so-called xe2x80x9cembossing diesxe2x80x9d which are used to emboss the diffraction structures into a lacquer layer in long runs. Such embossed holograms are usually prepared on a separate carrier and transferred by an adhesive layer to the final object to be protected against falsification, such as document, passport, credit card, CD, book, etc. The carrier layer can be removed from the layer structure of the hologram after the latter has been glued to the object to be protected.
The carrier material with the security device disposed thereon is frequently referred to as a transfer band and can be produced for example by the method known from U.S. Pat. No. 4,758,296. Therein, a sheetlike embossing die circulating on rollers is provided with a liquid resin and contacted with a plastic carrier material. During contact the liquid resin is cured by UV or electron beam. In a further step the relief structure is provided with a thin metal layer so that the information contained in the diffraction structures becomes visible in reflected light. For transfer of the diffraction structures to a security document, the layer structure is finally provided with a hot-melt adhesive layer which is activated under the action of heat and pressure.
However, it has turned out that transfer bands often have defects, i.e. places where no embossing lacquer is present or the embossing was not transferred sufficiently to the lacquer so that the brilliance of the diffraction effect is greatly restricted.
The invention is based on the problem of providing a method and apparatus which permit production of transfer bands without defects.
The invention is based on the finding that the various method steps during production of the transfer band place different requirements on the viscosity of the embossing lacquer, and therefore the viscosity must be adjusted in each method step and kept constant throughout the phase of operation for optimum results.
If the embossing lacquer is applied to the plastic foil by screen printing for example, viscosity must be between about 700 and 1500 mPas, by flexography about 100 to 500 mnPas or by gravure printing about 50 to 150 mpas, depending on the embossing lacquer used, to permit a uniform layer of desired thickness to be transferred to the foil. Which printing process is used depends on the layer thickness to be applied. Gravure printing can produce layer thicknesses of about 0.5 microns to about 3 microns, flexography about 3 microns to about 6 microns, and screen printing about 6 microns to about 12 microns. Since the lacquer is only applied in the form of a screen in printing operations, one must subsequently make sure the lacquer is distributed uniformly over the surface of the plastic foil. For this phase of operation the lacquer should be as fluid as possible; while for the embossing process a somewhat more viscous lacquer is advantageous. Hitherto the chemical composition of the lacquer and thus its viscosity were therefore selected so as to meet all requirements to a large extent. One had to accept the fact that the lacquer did not have the optimum viscosity for the particular phase of operation due to fluctuating working temperatures so that defects occurred in the finished product.
According to the invention, the viscosity of the lacquer is checked via a control unit in each phase of operation and adjusted to the optimum value for the particular phase. If lacquers with temperature-dependent viscosity are used, this control unit can consist of a temperature sensor and a heating or cooling assembly. The temperature sensor measures the temperature of the embossing lacquer continuously or at certain intervals and passes on the result to a regulating unit which controls the heating or cooling assembly in accordance with the measured temperature. The heating or cooling assembly can be in continuous operation so that the regulating unit merely provides a power increase or decrease, or the regulating unit causes a switch-on or switch-off of the assembly.